“The use of a highly lethal nerve agent against U.K. citizens on U.K. soil is an outrage. The attack was reckless, indiscriminate and irresponsible,” President Trump’s press secretary, Sarah Sanders, told reporters. “We offer the fullest condemnation and we extend our sympathy to the victims and their families and our support to the U.K. government. We stand by our closest ally and the special relationship that we have.”

Sanders spoke shortly after British Prime Minister Theresa May told British lawmakers that either the government of Vladimir Putin was directly behind the attack, or Moscow “lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others.” The attack targeted Sergei Skripal, who passed Russian secrets to the U.K., and his daughter Yulia.

May said that London had demanded an explanation from Moscow by Tuesday.

“Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom,” she said.

Asked whether the United States could corroborate the U.K.’s attribution, Sanders declined to do so, saying British officials were “still working through even some of the details of that.”

But, she said, U.S. authorities were “ready if we can be of any assistance.”

Sanders’s comments were the first time the White House had spoken publicly about the attack. A Yahoo News request for comment on Friday morning went unanswered.

It was not clear whether Trump and May had discussed the incident. Asked about transatlantic communications on the issue, a U.K. government spokesperson told Yahoo News that “there have been conversations between senior U.S. and U.K. officials. There will be more contact over the coming days.”